Novolak/diazonaphthoquinone (DNQ) based resists have been the most widely used positive-tone photoresists in the manufacture of semiconductors since their introduction in the 1960's. However, the demand for ever decreasing feature size in microelectronic devices has pushed the novolak/DNQ resists to their limits in terms of attainable resolution with conventional exposure techniques. Imaging resolution can be improved through the use of shorter wavelength radiation, such as deep-UV, electron beam, X-ray and ion-beam. However, these techniques are not applicable to current DNQ based resists. Among these new technologies, deep-UV imaging is presently believed to have the best potential for implementation in large scale production in the future.
In addition to improved resolution, it is also desirable to provide positive-tone resist materials having improved sensitivity. One approach to improving sensitivity uses the concept of chemical amplification. Chemical amplification involves the photogeneration within the resists of species that catalyze subsequent chemical events. One method of chemical amplification includes dissolution inhibition, wherein a masked phenol or protected carboxylic acid is mixed with a phenolic resin, resulting in a drastic decrease in the dissolution rate of the polymer in aqueous base developing solutions. A photoactivated acid-catalyzed deprotection reaction is then used to free the phenol or the carboxylic acid. As a result, the dissolution inhibitor is converted into a dissolution promoter in the radiation exposed areas of the resist material, allowing for the development of positive-tone images.
Previously, simple esters have been proposed as dissolution inhibitors in positive-tone photoresist applications. Examples are proposed in M. O'Brien, et al. Proc. SPIE 920:42 (1988) and E. Reichmanis, et al., J. Poly. Sci., Poly. Chem. Ed. 21:1075 (1983).
There remains a need in the art for a positive-tone photoresist formulation having both high resolution and high sensitivity. There also remains a need in the art for positive-tone photoresist formulations which are useful in deep-UV image resolution techniques. Moreover, there remains a need in the art for positive-tone photoresist dissolution inhibitors capable of providing high sensitivity.